For some time now, foams with a variety of different physical properties have been produced by the isocyanate polyaddition process from compounds containing several active hydrogen atoms, especially those compounds containing hydroxyl and/or carboxyl groups, and polyisocyanates, optionally in the presence of water, activators, emulsifiers, foam stabilizers and other additives (cf. R. Vieweg, A. Hochtlen, Kunststoff-Handbuch, Vol. VII, Polyurethane, Carl-Hanser-Verlag, Munich 1966).
By virtue of this process, it is possible, by suitably selecting the components, to obtain both elastic and rigid foams or any intermediate variants.
Soft-flexible polyurethane foams are widely used as upholstery materials in the manufacture of seat and back cushioning. Hard polyurethane foams are in demand as insulation materials against heat and cold, for example, in the refrigeration and building industries. In the form of structural foams (so-called "integral foams"), which have a densified to compact external structure, but have a cellular internal structure, hard foams are also used in the furniture and hobby industries.
Soft-flexible foams are expected to provide a high degree of comfort comparable with that of foams of natural latex or with that of a combination of standard upholstery materials, such as feathers, natural hair or wadding. Physically, the quality of the upholstery may be represented by the so-called "sag factor" (i.e. the quotient of the hardness value at 65% and 25% compression, the overall deflection being kept constant for 1 minute in either case) and the trend of the compression-deflection curves. To obtain good upholstery properties, the sag factor should have a value above 2.5, while the compression-deflection curves should not have any plateaux. In other words, minimal changes in deflection should not be accompanied by any significant change in deflection.
Another requirement which a foam intended for use in the upholstery industry has to satisfy is that its strength properties should be as high as possible to prevent tearing under mechanical stressing, such as may occur, for example, in the event of coating with textiles. Additionally, the hardness should be as high as possible for a given density (kg/m.sup.3). For economic reasons, foams with densities of from 15 to 30 kg/m.sup.3 are particularly preferred.
Hard polyurethane foams are expected to be as hard as possible, without being brittle, for the lowest possible densities. Tough foams are much more widely used than brittle hard foams in the industrial sector. For example, foams which are tough and flexible may be used in the manufacture of refrigerators and sandwich elements and in any applications where vibration is encountered, such as, in motor vehicles.
In order to satisfy these various requirements, attempts have already been made to use inert organic or inorganic fillers. Thus, it is known that solid additives may be introduced into one of the components used for foaming or directly into the foaming mixture, such as by directly introducing the fillers into the mixing chamber of a foaming machine. Attempts have also been made to carry out polymerization reactions in the polyols used for foaming. For example, dispersions of copolymers of styrene and acrylonitrile in polyethers containing hydroxyl groups have been used for this purpose.
Advantages of using fillers include greater hardness levels, increased elasticity in the case of soft foams, and reduction in the cost of the foam system according to the type of filler used. Unfortunately, these advantages are offset by serious disadvantages. Thus, it is difficult in cases where conventional fillers are used to prepare dispersions with such a fine grain distribution that storable mixtures are formed. The danger of sedimentation is always very pronounced, with the result that dispersions of this type have to be continuously stirred in order to eliminate the need for difficult re-dispersion operations. In cases where vinyl monomers are directly copolymerized in the polyols, the particles obtained are so small that there is generally no further tendency towards sedimentation. One disadvantage of these dispersions, however, is that they have to be free from monomers if they are to form substantially odorless foams which will be used for the typical applications of the foams. In order to insure that such dispersions are monomer free, the entire dispersion generally has to be passed through thin-layer distillation systems at elevated temperatures.
In fact, the copolymerization reaction itself is a critical step, since the radical initiators used in the polymerization of the vinyl compounds may increase the danger of chain cleavage in the polyethers. Not only may this interfere with the foaming process as a whole, but it may also result in the formation of foams with inferior properties. In the preparation of dispersions of this type, it is also necessary to use extremely pure monomers as starting materials in order to reduce internal discoloration of the foams as far as possible.
The use of fillers or of polyol dispersions containing fillers is attended by a number of disadvantages. Thus, fillers have a marked effect upon the mechanical properties of the foams. In general, the permanent set of soft foams is undesirably increased and their tensile strength and breaking elongation reduced, while the toughness of hard foams is reduced. This change in properties is particularly unpleasant in the case of highly flexible soft polyurethane foams because relative to these foams, the mechanical property level is already low.
According to German Offenlegungsschrift No. 2,110,055, highly flexible polyurethane foams may be obtained by reacting polyisocyanates with polyethers and low molecular weight crosslinking or chain-extending agents, such as aliphatic, cycloaliphatic or aromatic glycols, or amino-glycols or aliphatic, cycloaliphatic or aromatic amines, in the presence of blowing agents, catalysts and, optionally, other additives, but substantially in the absence of foam stabilizers of the polyetherpolysiloxane type.
Unfortunately, this process has considerable disadvantages. Thus, it cannot be used for the production of foams with a wide density range. Although it is possible by this process to produce foams with high densities, it is difficult, if not impossible, to reduce the density of the foams to values below 30 kg/m.sup.3. This is a considerable disadvantage for commercial and economical reasons because foams are generally handled according to volume and a certain volume should have as low a weight as possible.